The effects of man-made climate change, from sea-level rise to increasingly acidic ocean waters, have already become starkly apparent throughout the world. These effects are poised to worsen dramatically in coming decades due to continued emissions of greenhouse gases from the burning of fossil fuels and deforestation, according to a major new scientific report released on Sunday.

The report, which is the second installment of a three-part series of scientific updates from the U.N. Intergovernmental Panel on Climate Change (IPCC), sharply warns that climate change poses the greatest risks to the most vulnerable populations within all nations, and a potentially existential risk to poorer countries already struggling with food insecurity and civil conflict, as well as low-lying small island states.

According to the report, climate change is likely to ratchet up the amount of stress being placed on natural and human systems, to the point where increased loss of species is likely, along with increasingly frequent breakdowns in the functioning of human society.

In particular, the report cites the effects increased temperatures and heat waves have on essential food crops — in most cases lowering productivity — and warns of food availability and price swings that could lead to civil unrest in countries that are already having problems meeting the basic needs of their citizens. Climate change has already begun to hold back wheat and maize yields, the report found.

“Global temperature increases of 7.2 degrees Fahrenheit or more above late-20th century levels, combined with increasing food demand, would pose large risks to food security globally and regionally,” the report said.

The report says that the world is ill-prepared to manage the risks from a changing climate, and that if warming proceeds along the high end of the possible range of outcomes, climate change may overwhelm even the most well-prepared and wealthy nations. It paints a bleak picture of a hotter, more unstable future in which the combination of climate change and population growth combine to overwhelm the capacity of natural and human systems, resulting in increased poverty, conflict, and species extinction.

Societies can adapt to some of these impacts, the report says, but not all — especially if the amount and rate of warming during the 21st century turns out to be close to a worst-case scenario.

“With high levels of warming that result from continued growth in greenhouse gas emissions, risks will be challenging to manage, and even serious, sustained investments in adaptation will face limits,” said Chris Field, a scientist at the Carnegie Institution in California, who co-chaired the working group that produced the report.

“We’re not in an era where climate change is some sort of future hypothetical,” Field told reporters on Sunday. “There is no question that we live in a world that’s already altered by climate change.”

The report, which is the product of more than 300 lead authors and hundreds more contributing authors, focuses on climate change impacts as well as opportunities to adapt to a changing climate and reduce the global community’s vulnerability to climate change-related impacts. Such impacts include heavy precipitation events and coastal flooding.

In many cases, we are not prepared for the climate-related risks that we already face.

This report is part of the fifth major assessment of climate science from the IPCC, which is an intergovernmental body tasked with reviewing the state of climate science every several years, and issuing reports meant to inform policymakers. The first part of this latest IPCC assessment, on the physical evidence of climate change, was published in September 2013.

That report found that there is at least 95% certainty that humans have "been the dominant cause of the observed warming since the mid-20th century." It also projected global average surface temperatures are likely to exceed 2.7 degrees Fahrenheit above pre-industrial levels by the end of the century, and will likely range from 0.54 degrees Fahrenheit to 8.64 degrees Fahrenheit above 1986-2005 levels, depending on the sensitivity of the climate as well as the amount of greenhouse gas emissions.

With this new report, the IPCC states that climate change is best viewed as a risk management challenge. It also states with increased confidence that one of the significant risks related to climate change is more or worsened civil conflict.

"We live in an era of man-made climate change," said Vicente Barros, a professor at the University of Buenos Aires and co-chair of the IPCC working group that produced the report. "In many cases, we are not prepared for the climate-related risks that we already face. Investments in better preparation can pay dividends both for the present and for the future."

The IPCC report draws its conclusions from an exhaustive review of peer reviewed scientific papers published since the group’s fourth assessment report was released in 2007. According to the new report’s summary for policymakers, which was approved by government representatives and scientists meeting in Yokohama, Japan during the past week, the number of scientific publications pertaining to climate impacts, adaptation and vulnerability more than doubled between 2005 and 2010, reflecting the increased interest in the topic and the broad and sweeping changes observed in the climate system during that time period.

The findings make clear that vulnerability to climate change and exposure to its impacts varies in large part based on non-climatic factors, such as economic and political development. Poorer countries generally have more risk exposure and less capacity to adapt to climate change than wealthy nations, the report said.

The IPCC panel also found that evidence of climate change impacts is already being seen throughout the natural world, where species are shifting their ranges in order to keep up with a warming climate, and some species are dying out at a faster rate.

The IPCC report advises that climate change adaptation work needs to be pursued simultaneously with efforts to slash emissions, pointing to data showing that long-term warming will largely be determined by the amount of greenhouse gases put into the atmosphere during the next few decades. At the same time, the world is virtually guaranteed to experience continued climate change impacts during the next few decades due to the greenhouse gases that were previously emitted.

Each molecule of carbon dioxide, which is the most important long-lived manmade greenhouse gas, can remain in the atmosphere for as many as 1,000 years, making it more urgent to cut emissions in the near future, or face continued cumulative warming for centuries to come. The new report underscores the urgency of the task before policymakers around the world — take potentially expensive actions now to reduce emissions in order to avoid the worst effects of global warming years down the road.

"Climate change has already delivered severe economic damage and things will only get worse without more action," said Andrew Steer, the president of the World Resources Institute, a Washington-based think tank, in a press statement. "The report makes it clear that deep and rapid cuts in emissions can greatly reduce the costs of these impacts. Taking action now will undoubtedly be less expensive than waiting."

As previous reports have warned, this IPCC assessment found that higher levels of warming would increase the risks of "severe, pervasive, and irreversible impacts" from global warming, including species extinction and the loss of massive polar ice sheets that could raise global average sea levels by more than two feet.

The combination of high temperature and humidity in some areas for parts of the year is projected to compromise normal human activities, including growing food or working outdoors.

The report singles out coastal areas, including low-lying island nations, as hot spots of elevated risk that may not be completely manageable due to the steady climb in global sea levels projected to take place during the rest of this century, as the planet warms and land-based ice sheets melt.

"Some low-lying developing countries and small island states are expected to face very high impacts that, in some cases, could have associated damage and adaptation costs of several percentage points of GDP," the report said.

Coastal areas and nations dependent upon fishing for food and revenue are likely to suffer as well, the report said, with broad changes in marine species richness and fisheries catch potential as warming and increasingly acidic seas cause species to move to different parts of the ocean. Oceans are becoming more acidic as they absorb more carbon dioxide from the atmosphere.

The report sheds light on two contentious areas of climate impact studies, although it does not provide the definitive last word. In terms of the likely economic consequences of climate change, it found that additional temperature increases of 3.6 degrees Fahrenheit (compared to late 20th century temperatures) would cause global annual economic losses of between 0.2 to 2% of income, with losses more likely to be on the higher end of the scale. "Losses accelerate with greater warming," the report found.

Regarding human health impacts, the report found that through the middle of this century, most of the effects of climate change will be seen through the worsening of preexisting health problems, rather than the emergence of new diseases or spread of diseases to new areas.

At the high-end scenario of global warming, in which global average temperatures increase to 8.46 degrees Fahrenheit above 1986-2005 average levels by 2100, the report found that "the combination of high temperature and humidity in some areas for parts of the year is projected to compromise normal human activities, including growing food or working outdoors."

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